Supporting Information

Selenium-vacancies Rich CoSe2 Ultrathin Nanomeshes with

Abundant Active Sites for Electrocatalytic Oxygen Evolution

Yixin Zhang,a, ‡ Chao Zhang,a, ‡ Yamei Guo,a, * Dali Liu,a Yifu Yu,a, * and Bin Zhanga, b

a Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry,

School of Science, Tianjin University, No. 135 Yaguan Road, Haihe Education Park, Jinnan

District, Tianjin 300354, China

b Collaborative Innovation Center of Chemical Science and Engineering, No. 92 Weijin Road,

Nankai District, Tianjin 300072, China

* Corresponding Author: bzhang@tju.edu.cn (B.Z.)

‡ These authors contributed equally to this work.

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2019

1. Experimental Procedures

1.1 Materials Synthesis

1.1.1 Chemicals. Cobalt acetate (Co(Ac)2·4H2O, 98%), diethylenetriamine (DETA, 99%),

Potassium selenate (K2SeO3, 99%), cobalt powder (Co, 99.9%), Selenium powder (Se, 99.9%),

ethanol (99.7%), KOH (90%) and Nafion solution (5 wt %) were purchased from Aladdin Ltd.

(Shanghai, China). All chemicals are analytical grade and used as received without any

further purification.

1.1.2 Synthesis of lamellar CoSe2/DETA hybrid precursors. The lamellar CoSe2/DETA hybrid

precursors were synthesized according to the reported literature.1 In a typical procedure, 1

mmol Co(Ac)2⋅4H2O and 1mmol K2SeO3 were dissolved in 26 mL deionized water under the

condition of ice-water bath. Then, 13 mL diethylenetriamine (DETA) was dropwise added

into the solution. After stirring for half an hour, the obtained solution was transferred into a

50 mL Teflon-lined autoclave and maintained at 180 °C for 16 h. The black floccules were

collected after naturally cooling down to room temperature and then washed with absolute

ethanol. Finally, the black products were obtained after drying under vacuum at 60 °C for 6

h.

1.1.3 Synthesis of CoSe2 ultrathin nanomeshes with selenium vacancies (CoSe2 UNMvac).

The CoSe2/DETA hybrid precursors were put on the bottom of quartz boat and treated by Ar

plasma (commercial 13.56 MHz RF source) for 20 min with power of 300 W, pressure of 70

Pa and gas flow rate of 200 mL min-1. After half an hour, the sample was collected and

washed with absolute ethanol for once. Finally, the product was obtained after drying

under vacuum at 60 °C for 6 h.

1.1.4 Synthesis of CoSe2 nanosheets (CoSe2 NS). The CoSe2 NS were obtained according to

the reported literature by sonication treatment.1 In a typical procedure, 20 mg CoSe2/DETA

hybrid precursors was dispersed into 50 mL ethanol. Then, the mixture was sonicated at

300 W for 12 h. The resultant dispersions were centrifuged at 2000 r.p.m. for 10 min and

the precipitate was thrown away to remove the non-exfoliated component. The sample was

collected by centrifuging the suspension at 12000 r.p.m. for 10 min and washed with

ethanol. Finally, the product was obtained after drying under vacuum at 60 °C for 6 h.

1.1.5 Synthesis of CoSe2 nanoparticles (CoSe2 NP). The CoSe2 nanoparticles were

prepared according to the previous literature1. In a typical procedure, stoichiometric cobalt

powder and selenium powder were fully mixed and loaded in a fused silica tube. Hereafter,

the mixture was heated at 650 °C for 12 h in a static Ar atmosphere with a heating rate of

2°C min-1 and cooled to room temperature. The final sample was washed with distilled

water and ethanol for several times and dried under vacuum at 60 °C for 6 h.

1.2 Characterization

The scanning electron microscopy (SEM) images were taken with a Hitachi S-4800

scanning electron microscope. The transmission electron microscopy (TEM), high resolution

transmission electron microscopy (HRTEM) images, and elemental distribution mapping

images were carried out with a JEOL JEM-2100F microscope. The X-ray diffraction (XRD)

patterns were carried out with a Panalytical X'Pert Pro diffraction system with a Cu Kα

source (λ = 1.54056 Å). The thickness of nanosheets was determined by atomic force

microscopy (AFM) (Bruker multimode 8). Fourier transform infrared spectroscopy (FT-IR)

spectrum was carried out with a MAGNA1IR 750 (Nicolet Instrument Co) FTIR spectrometer.

The electron paramagnetic resonance (EPR) spectra were recorded on a Bruker EMX-8

spectrometer operated at 9.45 GHz 100 K. The X-ray photoelectron spectroscopy (XPS) was

recorded on Perkin Elmer PHI 1600 Versa Probe (Al Kα). All the peaks are calibrated with C

1s spectrum at binding energy of 284.8 eV.

1.3 g-factor in EPR.

The g-factor was calculated by the following formula.

𝑔 =ℎ𝜐𝛽𝐻

= 9.2710×10-21 erg·G-1; = 6.62620×10-27 erg·S; The electron paramagnetic resonance 𝛽 ℎ

(EPR) spectra were recorded on a Bruker EMX-8 spectrometer operated at 9.45 GHz 100 K,

so = 9.45 GHz = 9.45×109 Hz; We can obtain the from the Figure 2a, = 3373.7 G.𝜐 𝐻 𝐻

Therefore,

002.20020558.27.3373102710.9

1045.910626.6121

927

GGergHzSergg

1.4 Electrochemical measurements.

Electrochemical measurements were performed with a CHI 660D electrochemical

workstation (CH Instruments, Austin, TX) and a typical three-electrode cell was used,

including a working electrode, a Hg/HgO electrode (1.0 M KOH) as the reference electrode,

and a glassy carbon counter electrode in 1.0 M KOH electrolyte. The electrolyte was

degassed by bubbling O2 for 30 min before the OER measurements. If without extra

statement, all the mentioned potentials were against reversible hydrogen electrode (RHE).

All the data in electrochemical section was iR corrected. A glassy carbon electrode

decorated with catalyst samples was used as the working electrode. In a typical procedure

of the fabrication of the working electrode, 4 mg of catalysts and 20 μL of 5% Nafion

solution were dispersed in 1 mL deionized water by sonication to generate a homogeneous

ink. Then 5 μL of the dispersion (containing 20 μg catalyst) was transferred onto a glassy

carbon electrode of 3 mm in diameter (loading amount: 0.28 mg cm-2). The as-prepared

catalyst film was dried at room temperature. Polarization data were collected at a sweep

rate of 5 mV s-1.

1.5 Mass activity.

Mass activity (A g-1) of different samples as shown in Figure 3c was calculated from the

following equation.

𝑀𝑎𝑠𝑠 𝑎𝑐𝑡𝑖𝑣𝑖𝑡𝑦 =𝑗

𝑚

where m is the loading amount of electrocatalyst, i.e. 0.28 mg cm-2; j is the measured

current density (mA cm-2) at the overpotential of 200 mV, 250 mV, 300 mV and 350 mV.

1.6 The concentration of surface active sites from the redox features in cyclic voltammetry

(Figure S5).

Calculated area for CoSe2 UNMvac associated with the reduction of Co3+ to Co2+ =

0.0004697 V˙A

Hence, the surface concentration of Co for CoSe2 UNMvac that participated in OER =

0.0004697 V˙A / (0.3 V S-1×1.602×10-19 C×1)= 9.773×1015.

In the same way, the surface concentration of Co for CoSe2 NS = 0.0004632 V˙A / (0.3 V

S-1×1.602×10-19 C×1) = 9.638×1015.

The surface concentration of Co for CoSe2 NP = 2.5019×10-5 V˙A / (0.3 V S-

1×1.602×10-19 C×1) =5.205×1014.

1.7 Determination of Turnover Frequency (TOF) from OER current density.

TOF in our study was calculated assuming that the surface active Co atoms in the redox

reaction only participated in OER electrocatalysis. The following equation is,

FzNjTOF A

j stands for the current density (A cm-2), NA is the Avogadro number, z is the number of

electrons transferred to evolve a molecule of product (for O2, it is 4), F is the Faraday

constant (96485 C mol-1) and Γ is the surface concentration of active sites or number of

participating atoms in the catalyst material.

Therefore,

964854

10023.6 23jTOF

2. Results and Discussion

Fig. S1 Characterizations of CoSe2/DETA hybrid precursors. (a) SEM image and (b) XRD

pattern of lamellar CoSe2/DETA hybrid precursors.

The SEM image (Fig. S1a) clearly shows the lamellar two-dimensional (2D) morphology of

the as-prepared precursors. The XRD result (Fig. S1b) identifies the precursors as

CoSe2/DETA hybrids.

Fig. S2 Characterizations of CoSe2 NS. (a) TEM image, and (b) HRTEM image of CoSe2 NS.

After sonication treatment, the lamellar CoSe2/DETA precursors are exfoliated into free-

standing nanosheets as shown in TEM image (Fig. S2a). The HRTEM image (Fig. S2b) displays

the lattice spacing of 2.40 Å, which is corresponding to (211) crystallographic plane of cubic

CoSe2, indicating the formation of CoSe2 nanosheets (CoSe2 NS).

Fig. S3 FT-IR spectra of CoSe2 UNMvac, CoSe2 NS and CoSe2/DETA hybrid precursors.

The FT-IR adsorption peaks of organic group in CoSe2/DETA hybrids become disappeared

after exfoliation into CoSe2 UNMvac and CoSe2 NS (Fig. S3). These results demonstrate that

DETA is totally removed from lamellar CoSe2/DETA hybrid precursors after the plasma and

sonication treatment. The surface of as-obtained CoSe2 UNMvac and CoSe2 NS is clean, which

would be crucial for the following electrocatalytic measurements.

Fig. S4 Characterizations of CoSe2 NP. (a) SEM image and (b) XRD pattern of CoSe2 NP.

Table S1 Comparison of alkaline OER performance for CoSe2 UNMvac with other Co-based

electrocatalysts.

Overpotential

(mV)Catalyst Electrolyte

Mass

loading

(mg cm-2) η10

Tafel slope

(mV dec-1)

TOF (s-1)

η=300 mVRef.

CoSe2 UNMvac 1 M KOH 0.28 284 46.3 2.47 This work

CoSe2 NS 1 M KOH 0.28 343 97.7 0.51 This work

CoSe2 NP 1 M KOH 0.28 466 111.1 0.39 This work

α-Co4Fe(OH)x 1 M KOH 0.28 295 52 0.027 2

Co@NC 1 M KOH 0.32 390 3

Co3O4

mesoporous NTs0.1 M KOH 0.208 390 76 4

Co3O4 nanowires 1 M KOH 0.136 403 72 5

Co3O4 NA/CF 1 M KOH 1.9 71 0.19 at η=350 6

Co-CN SS 1 M KOH 0.285 340 79 7

CoMn LDH 1 M KOH 0.142 324 43 0.075 8

Co-MOF/NF 1 M KOH 5.84 77 ~0.013 9

CoP NPs 1 M KOH 0.71 340 99 10

FexCoy-ONSs 0.1 M KOH 0.36 308 36.8 0.022 at η=350 11

NiCo LDH 1 M KOH 0.07 ~335 41 0.11 12

CoCo LDH 1 M KOH 0.07 ~354 45 0.0035 12

NiCoP

nanoboxes1 M KOH 0.255 370 115 13

ultrathin porous

Co3O4

nanoplates

0.1 M KOH 0.56 523 710.0042 at

η=40014

η10: the overpotential corresponding to current density of 10 mA cm-2.

Areas are blank if the corresponding data are not mentioned in references.

Fig. S5 (a) Cyclic voltammetry (CV) curves of CoSe2 UNMvac, CoSe2 NS and CoSe2 NP. (b-d)

The area of redox features of different samples for the calculation of surface active sites.

Fig. S6 Characterizations of CoSe2 UNMvac after stability test. (a) TEM image and STEM-EDS

mapping images of CoSe2 UNMvac after stability test. (b) LSV curves of CoSe2 UNMvac. The

inset is the magnification of black box of CoSe2 UNMvac.(c) Co 2p XPS spectra of CoSe2

UNMvac after stability test. (d) Se 3d XPS spectra of CoSe2 UNMvac before and after stability

test.

CoSe2 UNMvac maintains the ultrathin nanomesh morphology after stability test (Fig. S6a).

The appearance of oxygen element in mapping images after stability test reveals the

transformation of CoSe2 UNMvac (Fig. S6a). The oxidation peak at ~1.22 V can be attributed

to the redox couple of Co2+/3+ (Fig. S6b), suggesting that the active species of CoSe2 UNMvac

might be CoOOH, which is oxidized from CoSe2. The Co-O peak at 780 eV in Co 2p XPS shows

the formation of CoOOH in CoSe2 UNMvac after stability test (Fig. S6c). Moreover, Co-Se

peak in Co 2p XPS (Fig. S6c) and the weak Se 3d peak (Fig. S6d) confirm the preservation of

part CoSe2 specie after stability test. All the results demonstrate that CoSe2 UNMvac is partly

converted into CoOOH during the OER process.

Fig. S7 Electrochemical capacitance measurements.

Electrochemical capacitance measurements were used to determine the electrochemical

active surface areas (ECSA) of catalysts. To measure the electrochemical capacitance, the

potential was swept between -0.02 and 0.08 V vs. Hg/HgO at different scan rates of 20, 40,

60, 80, 120 mV s-1. We measured the capacitive currents at a potential where no faradic

processes were observed, i.e. 0.03 V vs. Hg/HgO. The measured capacitive currents were

plotted as a function of scan rate in Fig. S8 and a linear fitting determined the specific

capacitance as 5.48 mF cm-2 for CoSe2 UNMvac, 1.18 mF cm-2 for CoSe2 NS and 0.0645 mF cm-

2 for CoSe2 NP after the stability test. The specific capacitance for a flat surface is generally

found to be in the range of 20-60 µF cm-2. In the following calculations of ECSA after

stability test, we assume specific capacitance as 40 µF cm-2.

222

2

137 40

48.52

ECSAECSA

UNMCoSeECSA cm

cmpercmFcmmFA vac

222

2

5.29 40

18.12

ECSAECSA

NSCoSeECSA cm

cmpercmFcmmFA

222

2

61.1 40

0645.02

ECSAECSA

NPCoSeECSA cm

cmpercmFcmmFA

The larger ECSA value of CoSe2 UNMvac demonstrate that there are more active sites in

CoSe2 UNMvac owing to the ultrathin nanomesh structure and the existence of selenium

vacancies.

Moreover, the specific capacitance is 2.78 mF cm-2 for CoSe2 UNMvac, 1.05 mF cm-2 for

CoSe2 NS and 0.0626 mF cm-2 for CoSe2 NP before stability test. Thus, the increment in ECSA

for the samples before and after stability test,

CoSe2 UNMvac : % 12.97

78.2 78.248.52

2

cmmFcmmF

CoSe2 NS : % 38.12

05.1 05.118.12

2

cmmFcmmF

CoSe2 NP : % 04.3

0626.0 0626.00645.0

2

2

cmmFcmmF

CoSe2 UNMvac have a maximum increment in ECSA with 97.12 % after the stability test,

indicating that the ultrathin nanomesh structure and the existence of selenium vacancies

would provide a huge increment of active sites in the oxygen evolution progress.

Fig. S8 Plots of the current density at 0.03 V vs scan rate for CoSe2 UNMvac, CoSe2 NS and CoSe2

NP after stability test.

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